1. Field of the Invention
The present invention relates to electro-dynamic transducer motor structures and, more particularly, to enhanced magnetic circuits in loudspeaker drivers.
2. Discussion of the Prior Art
Loudspeakers such as shown in commonly-owned U.S. Pat. No. 7,684,582 and in U.S. Pat. No. 7283,642 (Milot et al) convert electrical signal energy into acoustic energy by driving a diaphragm. FIG. 1 illustrates a prior art dome tweeter 11 (as disclosed in Milot's U.S. Pat. No. 7,283,642) using a moving-coil electrodynamic motor generally indicated at 10. The moving coil electrodynamic motor includes at least one permanent magnet 13 having two magnetic poles, a front plate or front pole piece 12 and a two-part rear pole piece having a proximal circular base 14 and a cylindrical, upwardly extending hollow distal projection 14′, with the annular permanent magnet 13 disposed between the front pole piece and the base of the rear pole piece. An annular magnetic gap 210 is defined between an inner circumferential edge of the front plate 12 and an outer circumferential edge of the sidewall of an annular distal projection 14′ of the rear pole piece 14. The front plate 12 and the rear pole piece 14 and its projection 14′ form a magnetic circuit which directs magnetic flux from the permanent magnet 13 into and across the magnetic gap 210. A dome-shaped tweeter diaphragm 16 is connected at its lower peripheral edge to a bobbin which carries a conductive voice coil 15 and is disposed in the annular magnetic gap 210 to allow the diaphragm to reciprocate along Z-axis 220 when driven.
The dome-shaped diaphragm 16 and the coil 15 of the Milot patent are connected to and supported by a surrounding chassis 17 by a peripheral suspension 18 to permit tweeter diaphragm movement. The distal projection or protrusion 14′ of the rear pole piece 14 is open at its center for the passage of conductive (positive and negative) audio signal leads 40 which are connected to the coil 15 to drive the diaphragm. The rear pole piece 14 and the front pole piece 12 may be annular, or ring shaped, are disposed at respective opposing poles of the magnet 13 and may be made of metal, such as soft iron. In this prior art tweeter, the magnetic circuit which directs flux from magnet 13 into annular magnetic gap 210 includes a first grooved surface 205 in the inner edge of the front plate 12 opposing a second grooved surface 206 in the outer circumferential edge of the sidewall of distal projection 14′, and those opposing grooved surfaces are intended to create distinct “zones” within magnetic gap 210, although the Milot patent does not exactly show how the magnetic flux lines are affected.
FIG. 2 illustrates in cross section another prior art loudspeaker motor structure 30, showing lines 32 of magnetic flux passing through parts of the motor's magnetic circuit, which includes a permanent magnet 34, a front plate 36, and a pole piece 38 having a distally (upwardly) projecting central segment 40 with an outer sidewall surface 42 defining part of a magnetic gap 44 between surface 42 and an inner circumferential surface 46 of the front plate 36. In the tweeter motor structure 30 of FIG. 2, the magnetic circuit which directs flux 32 from the annular permanent magnet 34 into the annular magnetic gap 44 includes the first surface 46 in the inner edge of the front plate 36 opposing the second surface 42 on the distal-most outer circumferential edge 42 of the sidewall of the rear pole piece's distal projection 40. The continuous lines of flux originate in the annular permanent magnet 34 and are enhanced by a proximal bucking magnet 105 located adjacent, and on the opposite side of, the rear pole piece 38. These lines of flux are shown as guided into and though the magnetic gap by the magnetic circuit. Typically, the pole piece of a well designed transducer motor is the choke point for the magnetic flux; that is, the pole piece, in this case pole piece 38, usually controls the overall reluctance. The amount of magnetic flux in the circuit is controlled by the amount of magnetic reluctance. Persons of skill in the art will recall that reluctance in a magnetic circuit is analogous to impedance in an electrical circuit. In a manner similar to the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance.
That the pole piece is the magnetic circuit choke point for a transducer motor is particularly true for tweeters such as those shown in FIGS. 1 and 2, where low diaphragm mass and small diameter are crucial. The flux-carrying capacity of the pole piece must be sufficient to maintain high levels of magnetic saturation, while the needs for small diameter and low mass place limits on the thickness of the front plate. High levels of saturation are necessary to reduce overall inductance and non-linear inductance (e.g., solenoid) effects in a tweeter, which matters because high inductance will reduce high frequency sensitivity. The pole reluctance limits both the useful gap width and the largest magnet size that can be effectively utilized, and these limiting factors limit tweeter performance. For example, Applicants have discovered that for a useful tweeter size which uses a 25 mm pole, the front plate thickness is limited to 3 mm to meet the saturation criteria and this limits the useful magnet diameter to approximately 70 mm. With only 3 mm of gap width, and a typical winding width of 1.5 mm, there is only 0.75 mm of total theoretical excursion in the motor. This excursion is severely reduced by both fringing flux and by the voice coil positional tolerance to a typical value of 0.3 mm.
There is a need, therefore, for a practical and effective structure and method to enhance the performance of magnetic circuits, especially those used loudspeaker motors such as tweeter motors.